But sometimes copyright assignment can be confusingly Machievllian, even in open source land.

Canonical, for instance, is one company that in the past has been knocked about on this issue. Developers and free software advocates have argued that it agreement, while simple and strightforward in form, actually contains what appear to be trapdoors. They're not hidden, either, as seen in Clause 6:

If copyrights didn't last so long, this might not be so big an issue - but he's right, copyright has plenty of loopholes and problems when it comes to giving back to the open-source community.